Supporting acting nominees are great because they range from a mind blowing, one or two scene performance (Ruby Dee) to a strong acting presence throughout the whole film (Amy Ryan, Tilda Swinton). This year there is really anyone's game, as there are five fantastic actresses nominated. The nominees are:

Cate Blanchett in I'm Not There
Ruby Dee in American Gangster
Saoirse Ronan in Atonement
Amy Ryan in Gone Baby Gone
Tilda Swinton in Michael Clayton

Prediction
Cate Blanchett. There's a lot of debate as to whether it will be her or Ruby Dee, but she literally transforms to play Jude, one of the Bob Dylan incarnates from I'm Not There, and probably the one closest to the actual Dylan. It was incredible to watch. Dee had all of three scenes, and only one in which you see real emotion, and I can't imagine the Academy awarding such a small performance.

What Should Win
Tilda Swinton. Her performance has been sorely overlooked, but is absolutely incredible. The film opens with her in a stall sweating profusely and looking terrified. Watching how she gets to this state is just amazing.